This invention relates to a control system for wind turbines and particularly to a control system that limits blade deflection to avoid tower strikes.
Wind turbines have long been used to generate electricity from the wind. The most common type of wind turbine is the horizontal axis turbine. Horizontal axis wind turbines have one or more blades (but most commonly 2 or 3 blades) attached to a shaft that rotates about a horizontal axis. On the opposite end of the shaft from the blades is a gearbox and generator. When wind passes over the blades, the shaft rotates and the generator makes electricity. Such wind turbines have been extensively used in California for the past 20 years and are being installed in large numbers all around the world.
One of the primary concerns in the design of a wind turbine is the cost of energy. In order to keep the cost of energy low the turbine must be rugged and reliable and have a low maintenance cost. Wind turbine manufacturers have improved the cost of energy by increasing the size of their wind turbines. Over the past 20 years, wind turbines on the commercial market have increased from approximately 50 kW in the early 1980s to nearly 2 MW today. As the turbine size and blade length increase, blade deflection becomes a more important issue. Some large wind turbines have been known to experience tower strikes in which a blade deflects to the point that it strikes the tower and is destroyed. Furthermore, many wind turbine manufacturers are reducing the cost of their wind turbines by making the blades lighter weight. This results in a more flexible blade and exacerbates the tower strike issue. Several wind turbine designs, such as those described in U.S. Pat. Nos. 4,352,629, 6,327,957 and 5,584,655 (all of which are incorporated herein by reference) describe highly flexible wind turbine blades. These highly flexible designs may become more common in the future as turbine designers strive to reduce the cost of energy even further. For flexible wind turbine blades to be successful on a large wind turbine, it is necessary to have a system to prevent tower strikes.